I am working on a customers vehicle which was started by another fellow 2 years ago. The vehicle is a '96 Silverado which was a 4.3/4L60e truck. The previous fellow installed a '05 LQ4 5.3L and a 4L60e out of a Tahoe. He totally cut the original ECM wiring out of the truck and purchased a stand alone harness and a 2000 Camaro SS 5.7/4L60e ECM. He cut all the grounds out of the truck and I have been replacing them as I found them. The engine and trans are wired and it is running, but when I attempted to drive the truck it wants to start off either in 2nd or 3rd. I am using the 2 connector range selector from the Silverado on the new transmission and everything wired up. The original cluster would light up the P, R,N,OD, D, 2,and1, but the Dakota digital dash the customer wanted installed will not show the ranges through the serial data input line (MIL). I think the wires are not connected to the ECM and will check this out. Will this cause my problem, or could I be missing a ground? I also cannot communicate with the transmission PIDs with my scanner. I have power at the trans, but I am worried about my grounds. Thanks for any advice you can give.

 

Aftermarket wiring harnesses... Jesus H. Christ. When will people learn?

Anyway, it sounds like the transmission is in "limp home" mode, where you only have 3rd gear and reverse. This MAY be because of a missing ground, bad internal harness, or something else.

Do you know which aftermarket harness he used?

 

No, I really don't. I have done a few LS swaps using a self made cut down original harness and the original ECM with VATS removed. Have even set up drive by wire and haven't had any problems. This is a new one on me. I looked up several harnesses on eBay and found one that matched the colors on the one I had. Used the schematic on that one and checked everything over. It looked right so I went with it. Engine runs great, only problem I had was he had bought a MAF with the IAT built in so I had to find a plug for it and rewire for it. It works. The ECM is wired for the gear positions (P,R,N,OD,D,2,1) but there is no plug for the 4 wire plug on the trans range sensor. I am going to wire from the range sensor to the ECM wires and see if that helps.

 

I didn't realize aftermarket harness's where so bad! He used an original harness and had no trans or o2 sensors.

 

The actually make an adapter harness for the built in IAT , I have one on my 95ss .

 

The PRNDL illumination comes from the NSS. The transmission is controlled by the PCM or another controller. Here is the way it works in a normal OE installation:

The ignition switch supplies power to the transmission fuse, usually a 10 amp fuse

The Trans fuse supplies the power to pin "E" and that is the power for the shift solenoids, TCC, 3-2 control & PWM TCC.

The PCM controls the ground for the shift solenoids pins "A" (1-2)- "B" (2-3), TCC pin "T", 3-2 control pin "S" & PWM TCC pin "U"

The EPC solenoid high is pin "C", solenoid low is pin "D".

Temp sensor high id pin "L", temp sensor low is pin "M"

The manual valve pressure swith uses Pins "N, P, R"

Here is a copy of TransGo's general reference data.

You can manually control the transmission through pins A, B & E by supplying power to pin "E" and grounding pins "A" & "B" in a specific way.
Ground both A & B for 1st.
Remove pin A from the ground for 2nd.
Remove pin B from ground and you have 3rd.
Re apply pin A to the ground to get 4th

If you have scan data you can look at data to see whats going on with the shift commands & EPC amperage. The PCM should show 1, 2, 3 & 4 for gear selection. The EPC amp data will be from 1.1A (low pressure)-.1A (high pressure).

If you have HPTuners you can use the VCM scanner to control the transmission in controls & special functions.

I hope this helps you diagnose your issue.

 

I used a Current Performance Wiring aftermarket harness for my personal LS swapped S10. It is the best harness out there. I had "0" issues with the harness. It was all plug and play. They make a stand alone harness as well and I'm sure that it's quality is top notch there too.
The problem I see is that people are focused on the $$$ and choose a cheap harness over a well made one.
I would definitely buy CPW again